This invention relates to methods for making depressed clad index optical fibers and is directed more specifically to techniques for preparing preforms prior to drawing optical fibers.
Depressed clad optical fibers were developed in the early 1980""s as an alternative to fibers with doped cores and less heavily doped, or undoped cladding. See, e.g., U.S. Pat. No. 4,439,007. Depressed cladding allows the use of fiber cores with relatively low doping. These cores provide low optical loss.
Applications have been developed for both single mode and multimode depressed clad fibers, and a variety of processes for the manufacture of depressed clad fibers were also developed. See e.g. U.S. Pat. No. 4,691,990, the disclosure of which is incorporated herein by reference. Complex refractive index profiles in optical fibers often use core regions of down doped silica.
Optical fibers with down doped regions have been found especially useful for lightwave systems in which control of non-linear effects is important. For example, in four-wave mixing of optical frequencies in the 1.5-1.6 mm wavelength region where DWDM networks operate, a low slope, low dispersion fiber is required. Recent advances in optical fiber technology have extended the DWDM range of operation to provide very high capacity transmission over a single fiber. Among these advances are fibers with nonzero-dispersion which are specifically designed to overcome pulse broadening and signal mixing in high power optically amplified DWDM systems over long distances. Current typical optically amplified DWDM systems operate in the 1530 to 1565 nm wavelength range, or the third window in the fiber spectrum. Emerging systems will use the fourth window (1565 to 1620 nm) to increase network capacity and optimize performance.
One of the important parameters in fiber designed for ultra high-speed networks is dispersion slope. Managing dispersion in the fiber itself reduces the need for high cost dispersion compensation components when used in high capacity WDM amplified systems.
Optical fibers for these and other advanced designs often require fiber cores with a down doped trench just outside the core region. These fibers have a modified W shaped index profile that has been found to be efficient for single mode guiding with low loss, and can be designed to have the dispersion characteristics mentioned above.
One technique for making depressed index regions in optical fiber preforms is to dope the region with fluorine or boron. In the case of a W shaped index fiber, the region of the core trench has a refractive index less than silica. In a preferred form, the center of the core is doped with, e.g. germania, to increase the index. In these structures, the xcex94n is relatively large, e.g. 0.005-0.010, between the center of the core and trench. The xcex94n between the trench and a silica cladding layer may be less than half that xcex94n value.
In the manufacture of preforms for these fibers, one approach is to down dope the outer layer of a porous core rod (the shell) by xe2x80x9csoakingxe2x80x9d the core rod in a fluorine containing gas atmosphere with the core rod still in the porous state, i.e. prior to consolidation. The porosity of the core rod at this stage in the process allows the fluorine gas to easily permeate the germania doped silica body. The porosity is typically in the range of 50-90%, measured as volume of solids to volume of voids. The conventional practice is to diffuse fluorine into the silica body using an equilibrium doping process. In this process, the silica body is heated to a temperature of rapid diffusion, in the presence of a low partial pressure of fluorine, i.e. a partial pressure sufficient to supply a continuous flux of fluorine to maintain the equilibrium diffusion. However, recognizing that the center portion of the core requires a higher index and cannot therefore be down doped, it has been difficult to confine the doping process to the core rod xe2x80x9cshellxe2x80x9d in a controllable and predictable way. Fiber preforms with down doped core regions and a center region doped with a conventional index increasing dopant such as germania allow some latitude in the selective doping of the shell. One approach has been to dope the center of the core with an excess of germania, and down dope the entire core rod with fluorine. This produces a depressed index profile in the shell, but at added cost and with added optical loss in the core.
Another approach to selective down doping of the core rod shell has been described by Kanamori et al., in U.S. Pat. No. 5,055,121. This approach uses a solid core rod onto which a soot layer is formed. The porous soot layer can then be doped with SiF4, which permeates the soot layer rapidly but diffuses slowly into the solid glass rod. In this way fluorine doping is confined to the shell region outside the core. This approach may be used for both down doping a shell region of a preform as well as for down doping a cladding of a preform. However, this approach is complex and expensive, requiring separate processing for the core rod, the shell, and the overcladding.
We have developed a fluorine doping process for optical fiber preforms with a xe2x80x9cWxe2x80x9d index profile that allows controlled doping of a porous silica core rod with fluorine but inhibits fluorine doping of the germania doped core. A key step in this process is a preliminary partial consolidation step wherein the germania core region is selectively consolidated prior to fluorine doping. The remainder of the core rod, still in a porous state, is doped with fluorine and then consolidated. The preliminary selective consolidation step protects the germania doped center region of the core rod from fluorine penetration. In a preferred embodiment of the invention, incremental fluorine doping is used. See U.S. patent application Ser. No. 09/755,914 filed Jan. 5, 2001. This fluorine doping process is relatively rapid, which further aids in preventing substantial fluorine doping of the center core region.